The present invention relates generally to liquid supply tanks and fill and overflow systems in such tanks. More specifically, it relates to a fill and overflow system in a water tank adapted for use as a mobile water supply suitable to assist fire fighting apparatus or the like.
In rural and other areas not adequately served by a fire hydrant or other adequate water supply, it is necessary to transport water to the scene of a fire by the use of a mobile water supply vehicle. Upon arriving at the fire, the water in the supply tank is emptied into a reservoir for future use, or it is pumped directly to the fire hoses for attacking the fire. After emptying, the water tank is returned to the water source for refilling and travels back to the fire if additional water is still required.
Factors affecting the utility of a water supply tank include how quickly the water may be unloaded at a fire, how quickly the tank may be refilled, and the speed with which the transporting vehicle can shuttle between the water source and the fire. One known method of increasing the tank filling and evacuation speed is to provide a free breathing tank with uncapped venting ports. This construction allows a free flow of air between the tank and the atmosphere, thereby minimizing the partial vacuum formed within the tank chamber during evacuation which restricts the speed of water evacuation, and minimizing the pressure buildup within the tank chamber during filling which restricts the speed of filling. Such a free breathing construction also minimizes stress on the tank itself resulting from the alternate positive and negative pressures experienced by the tank as the chamber walls are placed under tension or compression. During maneuvering of the transporting vehicle en route to the fire, however, this open vent construction allows water to spill from the venting ports of the water supply tank, especially during high performance maneuvering. This situation is hazardous since water spilled in front of the traction wheels of the water supply vehicle could cause a skid or loss of traction. Spilled water also endangers the general road traffic. And in cold weather, water spill is particularly hazardous as freezing temperatures cause the water to form sheets of ice on the road surface, creating extremely poor driving conditions. The combination of water spillage and freezing temperatures may also cause controls on the external surface of the tank to freeze or become ice covered. If the tank is used to supply liquids for other uses, spillage poses even further problems and complications if the liquid in the tank is other than water, such as a corrosive or a petroleum product.
To prevent liquid spillage during maneuvering, it is common to provide the supply tank with lockable venting ports that may be closed during transport. This system introduces a delay in the filling and evacuation operations, however, as it requires the vents to be opened before the tank may be filled or emptied. And, if the vents are not opened, the filling and evacuation operations are slowed due to the abnormal pressure or vacuum created within the tank, as explained above. Further, structural deformation of the tank may occur during filling or evacuation as the pressure or vacuum buildup within the tank causes the tank to balloon or collapse when the venting ports are closed, preventing release of excess pressure and liquid overflow or the admission of atmospheric air.